Final answer:
To encrypt the message 'HideatFirstandHackatmidnight' using rows 8 characters long, arrange the letters in a grid and read them column by column.
Step-by-step explanation:
To encrypt the message 'HideatFirstandHackatmidnight' using rows 8 characters long, you can arrange the letters of the message in a grid with 8 columns. If the message doesn't evenly fill the grid, you can pad it with additional characters. Then, read the letters column by column to get the encrypted message.
For example, if we arrange the message as follows:
H I D E A T F
I R S T A N D
H A C K A T M
I D N I G H T
The encrypted message will be: 'HIAIRHDNDAEEKCITTNFTAG'.